Beside the leaking tomb

The wide plain north of Kiev is snowbound, studded with birch and pine forests. In a few weeks it will burst with flowers and life but now the rivers are frozen and a bitter wind blows in from the steppes. After two hours, the road stops at a high fence. It looks like an old Soviet border crossing, but it's the first checkpoint of the Chernobyl exclusion zone. We are still 30km from the nuclear reactor that blew apart during a routine safety test on April 25 1986 and scattered radiation equivalent to 270 Hiroshima bombs over much of the northern hemisphere.

The first signs of the world's largest man-made disaster are small heaps of earth, like burial mounds, on the roadside. Each has a pole with a radiation sign on it and represents one house. To our left is a forest of markers, to our right a crumbling village of boarded up houses. When the "liquidators", the 600,000 people who over seven years came from all over the former Soviet Union to clean up the site, first arrived, they started to tear down the contaminated villages. Then they found that the dust they were creating was itself a threat so they left them. More than 115,000 people were evacuated; two towns, and 74 villages.

In the uninhabitable village of Illintsy, the radiation level is 2.0. Yet a few yards away it's 3.2. Wherever the explosion and the wind took the radioactive particles is now a hot spot.

The road is crumbling and hasn't been repaired in 20 years. Behind us is the "red" forest. Thousands of trees turned crimson after the blast and had to be felled and buried. A few miles away is a four kilometre by two kilometre field of helicopters, buses, concrete mixers lorries and hardware used in the clean-up. All are highly irradiated and have been left, perhaps to be buried in years to come.

A mile further on and we get the big picture. Chernobyl was not just one nuclear power station to serve a region, but a complex of four, like having Sellafield, Hinckley Point, Dounreay and Dungeness all on one vast site. Two more nuclear reactors were being built when the explosion happened. Work stopped immediately. They left the cranes, the unfinished buildings, the half-constructed cooling tower, everything.

We approach the stricken reactor no 4 from the rear. We are told to stay behind a wall, to move only where the radiation meters say it is safe. The building towers above us just 200 metres away but is curiously unimpressive and featureless. Its top and one whole side were blown out in the blast and fires raged for days. They tried dropping lead and water on the fires but it made them worse. The only way to stop the radiation in 1986 was to encase the whole edifice in concrete. Armies of soldiers and volunteers from all over the Soviet union came and in just seven months mixed 410,000 cubic metres of concrete; they brought in 400 miners and built a giant plinth below the reactor to stop the radiation polluting the water courses.

Reactor 4 is now called the "sarcophagus", but this tomb is leaking radiation. Inside are hundreds of tonnes of unstable nuclear fuel, tens of thousands of gallons of highly radioactive water. The whole building tilts and is in theoretical danger of falling. A new solution is needed and an international consortium of engineers has come up with the idea of a giant arch to cover the whole reactor. It would measure 250 metres by 150 metres and be 108 metres high, made of 6ft thick steel. It would cost $1.8bn, but it would only last 100 years.

When the building blew, radiation levels were up to 20,000 millisieverts. The army conscripts were offered a year off their military service for every minute they spent in the danger zone. Now the monitor jumps to seven, eight, nine and then 10 millisieverts. We are only allowed two minutes.